For measuring a target point or, in particular, a plurality of target points, numerous geodetic surveying devices have been known since antiquity. In this case, distance and direction or angle and from a measuring device to the target point to be measured are recorded and, in particular, the absolute position of the measuring device together with reference points possibly present are detected as spatial standard data.
Well-known examples of such geodetic surveying devices include a theodolite, tachymeter and a total station, which is also referred to as electronic tachymeter or computer tachymeter. One geodetic measuring device from the prior art is described in the publication document EP 1 686 350, for example. Such devices have electrical-sensor-based angle and distance measuring functions that permit direction and distance to be determined with respect to a selected target. In this case, the angle and distance variables are determined in the internal reference system of the device and, if appropriate, also have to be combined with an external reference system for absolute position determination.
In many geodetic applications, there is surveying of points by virtue of specially embodied target objects being placed there. These usually consist of a plumb rod with a module that can be sighted, for example with a reflector for defining the measurement path or the measurement point. These target objects are sighted by means of a surveying device, a direction and a distance to the objects is determined and hence a position of the object is derived.
Analogously to this surveying of a point, there can be defining or marking of target points already known in terms of coordinates or of points, the positions of which were defined in the run-up to a marking procedure. In contrast to the surveying of points, the position or the coordinates of the points to be defined are known and to be marked in this case. A plumb rod or a surveying rod, which is guided by a user and positioned on a target point, is usually likewise used for such a procedure. For this, the user can approach the target position of the target point on the basis of position information generated by the surveying device, wherein the surveying rod is sighted by the surveying device, either by a second person or automatically by automation assigned to the surveying device. Once the target point has been reached, the user can undertake marking of the point.
Modern surveying devices, such as a total station, have microprocessors for digital processing and storing of captured measurement data. The devices are generally produced with a compact and integrated configuration, wherein coaxial distance elements and angle measurement elements, which are usually coaxial, and also computing, control and storage units are integrated in one device. Depending on the configuration level of the total station, means for motorizing the target optical unit, for reflector-less distance measuring, for automated target search and tracking and for remote control of the whole device are integrated.
Total stations known from the prior art moreover have a radio data interface for establishing a radio link to external peripheral components such as e.g. a data capture device, which, in particular, may be embodied as a portable data logger, remote control unit, field computer, notebook, minicomputer or PDA. By means of the data interface, it is possible to output measurement data captured and stored by the total station for external processing, to read externally captured measurement data into the total station for storage and/or processing, to input and/or output remote control signals for remotely controlling the total station or a further external component, in particular during mobile field use, and to load control software into the total station.
For sighting or targeting the target point to be surveyed, geodetic surveying devices of the generic type for example have a telescopic sight, such as e.g. an optical telescope, as sighting device. The telescopic sight is generally rotatable about a vertical axis and about a horizontal tilting axis relative to a base of the measuring device, such that the telescopic sight can be aligned with the point to be surveyed by pivoting and tilting. Modern devices can have, in addition to the optical viewing channel, a camera for capturing an image, said camera being integrated into the telescopic sight and being aligned for example coaxially or in a parallel fashion, wherein the captured image can be displayed, in particular, as a live image on the display of the display control unit and/or on a display of the peripheral device—such as e.g. the data logger or the remote control unit—used for remote control. In this case, the optical unit of the sighting device can have a manual focus—for example an adjusting screw for altering the position of a focusing optical unit—or have an autofocus, wherein the focus position is altered e.g. by servomotors or it has fixed focusing in conjunction with a wide-angle lens. Automatic focusing devices for telescopic sights of geodetic devices are known e.g. from DE 197 107 22, DE 199 267 06 or DE 199 495 80.
On an image captured by the camera, it is moreover possible—in accordance with known embodiments of generic surveying devices—to insert superposed information in respect of target points, the points themselves, object structures and further object data. By way of example, this data can originate from a three-dimensional terrain model. By means of such provision of design data on a display together with the captured image, it is possible to carry out controlling or monitoring of construction advances. Such an electronic surveying device with an image capture unit and an output unit for displaying design data is known for example from EP 1 496 281.
In the case of such a display of object data in an image captured by a camera, the number of objects or amount of information in relation to objects shown in the image may vary strongly depending on the respective measurement condition. By way of example, if the viewing field of the camera is aligned in a direction in which a multiplicity of stored object data lie in the viewing field, a superposed display of the information can appear very confusing, in particular to a user, due to many intersecting symbols.
Accordingly, in respect of the prior art, what is disadvantageous in superposed display forms is that although much information or many structures are displayed simultaneously in a camera image, an operator of the surveying system only requires a small proportion of the wealth of information on show for carrying out his measuring object. Until now, the operator in such a case had to laboriously identify the object data in the image relevant to him. Within the scope of the measuring process, this procedure means much time is spent and it moreover harbors that error source of the operator of the system selecting information differing from the desired information as a result of the multiplicity of displayed object data. Furthermore, objects captured in the image may be covered by the information displayed in the image or may not be identifiable, even though the image objects should perspectively be in the image foreground.